1. Field of the Invention
This invention relates to methods and apparatus for signalling between a first terminal and a second, via a network employing a signalling protocol.
2. Related Art
One such access signalling protocol is defined in ITU Q2931, entitled “Broadband Integrated Services Digital Network (B-ISDN)—Digital Subscriber Signalling System No 2 (DSS 2)—User-Network Interface (UNI) Layer 3 Specification for Basic Call/Connection Control”, well known, and available from the International Telecommunications Union, of Geneva, Switzerland.
This protocol defines the signalling procedures to be followed by a first terminal in accessing a second to set up and operate an ISDN communications session.
Many conditions may exist or occur during such a communications session. For example, the remote terminal may be busy, or unable to signal at a certain rate, or unobtainable (damaged or switched off). Alternatively, the network may be congested, or a busy tone may be generated from the local exchange. Data may be lost, or delayed, or corrupted in passage.
Many such protocols, such as Q2931, define a state machine; that is to say, a machine that can exist only in one of a number of predefined states, and can move from state to state in response to the occurrence of a predetermined event or combination of events. Actions are performed in moving the terminal from one state to another.
Such events may be, for example, the receipt of a defined signal from the second terminal via the network, or the elapsing of a predetermined time in the state. The actions taken in response may involve external signalling, or resetting an internal timer, and moving to a new state. The protocol defines the set of states, and the set of actions, and the events triggering the performance of the actions and changes of state.
A simple call set-up sequence starts with a first terminal moving from the on-hook state to a first active state in response to user action, dialling a second terminal, and then entering a wait state awaiting the next event. The two terminals pass through a succession of such states during handshaking signalling, until the call is set up.
If unusual conditions in the network or at either terminal occur, however, it is necessary to use appropriate exception-handling behaviour, or else one or both terminals may “hang”, awaiting a “normal” event which will never occur. Such conditions will be interpreted as events, corresponding either to receipt of a “signal” (which may be noise) or elapsing of a time period within which the normal signal should have been received.
The total number of such unusual conditions is large, and each may require different handling depending on the state the terminal is in at the time. This leads to a large number of responses in a set such as Q2931. Encoding such protocols as an operating control program for a terminal may require, for Q2931, 200 kilobytes to 1 Megabyte of executable code.
It might be supposed that the widespread availability of cheap memory and disk drive components would readily accommodate a program of such a size.